Genetic engineering of forest tree species to conform to desired traits has shifted the emphasis in forest tree improvement away from traditional breeding programs. Although research on genetic engineering of forest trees has been vigorous, the progress has been slow due either to a lack of an efficient gene transfer mechanism or lack of an in vitro culture system for plant regeneration.
Of Populus species, quaking aspen is one of the most commonly used species for wood pulp production in North America due to its good fiber properties, fast growth, and world-wide distribution. In addition to these characteristics desired for a pulpwood species, it is economically beneficial to genetically engineer new traits into Populus species such as quaking aspen such as altered lignin composition.
The only known Agrobacterium-mediated transformation and regeneration of quaking aspen was reported in 1986 using hypocotyl and leaf segments of in vitro plants derived from seedlings. However, this and other techniques developed for Populus species are not capable of regenerating whole plant from plant tissue derived from field-grown and greenhouse-grown plants or require tissue from in vitro plants. It is known that whole plant regeneration from greenhouse-grown plant materials is more difficult than that from in vitro plant materials or embryonic materials including mature and immature embryo, cotyledon, hypocotyl, and seedling.
A transformation and regeneration method for greenhouse and field-grown plants is needed for Populus species such as quaking aspen. The continual availability of field-grown as well as greenhouse-grown plants through vegetative propagation and the ease of maintaining these plants are the obvious advantages of using greenhouse and field-grown plants over the use of in vitro plants derived from seedlings. However, the difficulty of shoot regeneration from greenhouse and field-grown plants has been one of the main reasons that in vitro plants from seedlings have been the preferred explant materials.
With respect to lignin, angiosperm lignin also called guaiacyl-syringyl lignin is formed from the polymerization of two main precursors: coniferyl alcohol and sinapyl alcohol. The ratio of syringyl to guaiacyl units is directly related to the efficiency of kraft delignification, with higher syringyl quantities improving the pulping efficiency. There is a need for altering the lignin composition in that it is economically beneficial to genetically alter lignin composition to increase the ratio of syringyl to guaiacyl units for some applications and reduce the ratio for other applications. For example, pulping by the paper industry would be made more efficient with plants having a higher ratio of syringyl to guaiacyl units and novel wood would be available having a lower ratio of syringyl to guaiacyl units.